You can now buy a battery/alternator tester for under $25 at places like Harbor Freight. They are handheld with 2 big jumper clamps and a couple of rocker switches. We use one at the shop all the time.
or.....
Place a volt meter (don't have one?? go buy one, cause you'll need one the rest of your life)....across battery terminals and read voltage. S/B 12 to 13.5v.....start car and measure alternator output. S/B 13v-14v
NO??? then alternator is not keeping the battery up. Replace alternator (and maybe battery too, as batteries need to stay charged).
Yes?? turn off car, then take the negative terminal off the battery and place the volt meter leads between the battery post and the terminal.
Are you reading any voltage?? More than like 0.5 volt?? No? then there is not enough drain to cause your problem...drop by a shop to have the battery load tested.
Yes?? then start pulling fuses until the draw stops. That will be the circuit that is draining the battery.
Now....we just had a Honda in here that had an "intermittant" problem with the 'main relay' (which also is the fuel pump relay). It would stay on sometimes (like over night) and drain the battery, and if she did not start it for 2 days, the battery would be to low to crank it over. We finally discovered it because the relay was 'hot' one time while we were trying to diagnose it. New main relay took care of that.
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